The invention relates to a method for strip-coating a metallic strip-shaped substrate with a thin strip of plastic, an apparatus for carrying out the method as well as to the coated strip obtained with the method.
There are at least two methods known for manufacturing a coated product comprising a metal substrate and a plastic layer adhering to it, namely film-laminating and extrusion-coating.
In the case of film-laminating, a finished plastic film is unrolled and applied onto the metal substrate, as disclosed in e.g. WO 93/24324.
In the case of extrusion-coating a sheet of plastic is applied onto the metal substrate directly or virtually directly from an extruder, as disclosed in e.g. EP 0 067 060 Al.
In the case of the first method a roll of finished film is taken as stag material, A problem in making a roll of film is rolling it up. The film tends to stick to itself so that the windings cling to each other. Because in its rolled up state the film shrinks somewhat the roll has to be rolled up loosely to enable it to be unrolled once again in a controlled way. Inevitable stresses in the film then easily cause edge build-up, the roll becomes unround, and the film displays spacing tracks when being unrolled. Among other things this makes the film incapable of being unrolled without difficulty at a sufficiently high speed; if is does succeed then there remains the problem that at higher rolling off speeds electrostatic discharge symptoms need to be reckoned with. To avoid such difficulties additives are added to for example household foils; in the case of film-lamination this solution offers no remedy because the additives unacceptably reduce the capacity to adhere to the metal substrate.
On the face of it extrusion-coating would therefore seem an interesting alternative, and so it is for a small number of applications, namely those whereby the plastic involved has the correct adhesion properties in molten state. When this is no longer the cases or not adequately so, and molecules need to be incorporated in the plastic to migrate to the surface in order to accomplish adhesion, in the case of extrusion-coating the problems occur, at least where a high coating speed is desired. This is because adhesion groups only migrate fast enough, i.e. within tenths of a second, if a sufficiently high temperature can be maintained dig the coating. This is only possible when coating onto one side of the substrate, The required high temperature then also makes it impossible subsequently to coat the other side because the previously applied coating becomes unacceptably damaged on the second exposure to the high temperature. Even non-subsequent but simultaneous two-sided extrusion-coating is no solution because in the case of extrusion-coating the slightest deviation in substrate thickness and the slightest disturbance in the process would cause unstable process operation and consequently coating differences and inhomogeneities on each side.
The problems surrounding the procedures outlined are resolved or at least largely reduced if worked in accordance with the invention.
The method in accordance with the invention is characterized in that it comprises in combination the stages
(i) in-situ casting of a plastic strip;
(ii) leading the plastic strip around a preferably initially water-cooled cooling roll;
(iii) leading away the plastic strip until the plastic strip production is underway and stabilised;
(iv) bringing the plastic strip and the substrate up to speed and heating the substrate to a temper of the substrate close to or above the softening temperature of the part of the plastic strip facing the substrate;
(v) pressing the plastic strip onto the substrate and where applicable breaking off the plastic strip and stopping it being led away, while the substrate and the cooling roll are connected by the plastic strip;
(vi) coating the substrate with the plastic strip at high speed
This achieves the effect of enabling a considerably inner plastic layer to be applied onto the metal strip in a controlled and economically viable manner.
It is remarked that U.S. Pat. No. 5,407,702 discloses a method for coating a metal strip with a polymer extrudate which extrudate after extrusion is firstly brought into contact with a surface having a temperature which will promote sticking or clinging of the extrudate thereto. A typical temperature for this purpose is said to be in the range of about 120xc2x0 C. to 180xc2x0 C.
The invention is also embodied in an apparatus for the continuous strip-coating of a metal substrate with a layer of plastic.
Finally the invention is further embodied in a strip-coated packaging steel.
The invention will now be further illustrated by reference to the drawing comprising FIGS. 1, 2 and 3 each of which show a possible line drawing for coating in accordance with the invention, and several non-limitative examples with references to the Figures.